UPS sizing warning

This is slightly o.t but I know that a lot of us install UPS's in front of PLC's so I thought it was relevant.

We have a very expensive X-ray unit that has been giving us problems. We had a service tech out last week that looked at the incoming 120V supply and complained about it's quality. To be honest it was pretty ugly.

Although I didn't think that this was causing a problem with the machine I decided to install a double conversion online UPS to provide a "perfect" sine wave to his machine.

The X-ray machine was rated at 900VA max input. I put an amp clamp on the line while turning it on and observed about 8A inrush and 5A running current.

The UPS I spec'd was rated at 1000VA running and 110% overload for 3 minutes. Should be OK right??

Today I hooked up the UPS to the machine only to find that it immedeiatly went to overload when the X-ray was turned on. I phone the UPS manufacturer to find that the UPS was undersized for the application. Turns out the UPS is rated at 1000VA but only 700W because of the internal PF of the UPS and therefore could only provide 5.8A output.

Now I'm MAD.

I understand the difference between a watt, a volt amp, and a var. I understand the power triangle.

So how do these guys get away with this FALSE ADVERTISING. When you buy a transformer you spec it in volt-amps, knowing that the actual "power" developed is dependant on the power factor of your load. When you spec a motor you use horsepower or KW rating knowing that the draw of the motor is determined by the efficiency and power factor of the motor.

Now I have two problems, I look like an idiot in front of my new boss. More importantly the service rep from the X-ray manufacturer is going to be here on monday and I still have ugly power.

I'd love to slam the manufacturer but upon further investigation I found that ALL UPS manufacturers market their product this way. If this were a washing machine there would be a class action law suit against the manufacturer for false advertising. I offer this post as a warning to others so they do not get into the same situation.

Or am I an idiot and eveyone knew this?????

I didnt know this.

Thanks

Sounds like UPS's are spec'ed the same way stereo amplifiers are.


I agree that is annoying but am glad to know before I went down that road.

I still refuse to buy Tripplite brand because they were notorious for amrketing power ratings that were not what one would expect. I bought a power supply for a ham radio and discovered the rating was not a standard rating, so I put 'em on my personal banned list.

I remember that Radio Shack used to have a "music watts" rating for their H-Fi amps that was aloowed for X% overmodulation, or some such ridiculous definition.

It so happens I did know the Gotcha on UPSs.

Tell you boss they fudged the definitions and the whole industry succumbed to deceit as a way of business.

Dan

That's good info to know.

I'll have to file it away in the brain bank and hope I remember it when the time comes.

Thanks for sharing.

It is unfortunate but yes i did know and you are right..all UPS people size there units this way..never looked into why..Have you ever brought a vacuum cleaner?? There is a dirt devil out there with a 5HP motor..or so they want you to think..do the math..a 5HP at 110 volt wold dim the lights and blow the main breaker!!..Not realy but you get the point..a lot of people sell stuff like this..HIFI equiptment was mentioned..you will see a lot of amps that say 400w and in small print beside (Peak) or (DIN)..what you want to look for is the RMS..

Anyway you are right it sucks..

I know about false ratings, my vacuum has 20 Amps of "cleaning power" and my air compressor is rated at 5 HP "peak". both plug into a 15 amp circuit.

My dissapointment is mostly because these are high end UPS's, and their soul purpose is to provide POWER. I would hope that companies marketing industrial electronics would have more ethics, I guess I was wrong.

So now I tried to "overnight" a 1500VA unit (to power my 900VA load that couldn't be powered by a 1000VA supply). The courier messed up and dropped it off about 500 miles from me and I still don't have it.

When it rains it pours.

I've inadvertently been sizing mine off the wattage rating and thus never had a problem. I just never noticed the VA rating. I think it's what they call dumb luck.

If the power factor is around 60%, is the 1.5kVA going to be enough? (1.5kVA/120V)*.6 = 7.5A Or does the larger unit have a higher power factor?

edited to add that I'm tired and it looks like the power factor is actually closer to 70%.

The larger unit has a rating of 1050 watts, making the power factor 0.7. I gave the UPS manufacturer the specs on our machine and the readings I had taken and was assured that it was big enough and would have no problems.

I hope he is right.

allscott said:

This is slightly o.t but I know that a lot of us install UPS's in front of PLC's so I thought it was relevant.

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Why do you put a UPS in front of a plc ? I'm interested as I've not come across this before.

That was the opening statement but the application was to clean the power in front an X-ray system. Personally do not understand why you would ask that question, there are many reasons to use a UPS in front of a PLC.

It is not false advertising, the device capabilities depend on the system requirements. It is up to the person needing/ordering the device to determine if he is getting what he needs.

Knowing about electrical terms etc does not always mean you know when to apply them correctly or use the proper formulas involved. Undoubtedly there is more to this system then you originally thought that required you to match it using wattage instead of VA.

You are blaming the UPS manufacturers for false advertisement, I have visited the 3 major sites and all show VA and Watts for their components. Just one look at a 1000VA/700W unit would make me attempt to understand why there is that great a difference.

SimonGoldsworthy said:

I'm interested as I've not come across this before.

Click to expand...

Is this too obtuse then ?

SimonGoldsworthy said:

Is this too obtuse then ?

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I dunno, could be scalene or equilateral.

Hi Simon.

I have used the dedicated ups from siemens, it’s a battery pack and control card. I needed it in order to send a power failure sms from the plc.

Without it there was no power to complete the alarm sms….lol.



In my opinion using a ups before any electronic device is a good idea in general, but that’s just me.



Keep well.

SimonGoldsworthy said:

Why do you put a UPS in front of a plc ? I'm interested as I've not come across this before.

Click to expand...

As DDV mentioned, there is the power outage alarm.

How do you want the PLC to react to the power being out for 1sec, 1 minute, 10 minutes. It is a lot easier to know how restart if the PLC knows how long the system has been down. Timers don't run with power to the PLC.

Another reason is that PLCs are sometimes used as data acquisiton devices as well as control. During batch processing, if you have data to tell you about the product during the power outage, you may be able to confidently say the parts are okay (or NOT). It's a much easier sell to QC than just saying "even though there was a XX minute gap in data..."